1. Field of the Invention
This invention relates broadly to a method and apparatus for determining coil sheet length. As described below, the invention may be used in a metallic sheet rolling mill to provide a highly accurate computer method and apparatus for determing coil sheet length. In addition, the invention may also be used in the production or processing facilities of other sheet materials, such as in the paper and plastic industries.
2. Description of the Prior Art
In the steel industry a wide range of sheet materials having various thicknesses, widths, composition and finishes are produced on high speed rolling mills. Frequently, the rolled sheet product is wrapped in large diameter coils to await shipment. Heretofore, customer billing practices were based on actual weight of the coil sheet product when shipped, there being no assurance to the customer as to actual coil sheet length. Industry records have shown that variations in production practices have caused significant variations in coil sheet length for comparable orders.
Currently, however, a theoretical weight billing practice is in effect for coil sheet steel products. That is, in one case the billing of steel coil sheet is based on a weight calculated from a customer's ordered thickness, width, composition and length, the latter being indicated by the steel producer's rolling mill coil sheet length measurements. Under the theoretical weight billing practice, coil sheet length shortages are debited to the producer, and sheet length overages are credited to him. In practice, however, coil shortage debits are claimed more frequently than credits allowed for coil length overages in the absence of accurate coil length records. Therefore, both coil sheet length shortages as well as overages are in reality a liability to the coil sheet steel producer. Hence it behooves the steel producer to procure and maintain accurate coil sheet length measuring apparatus to minimize such liabilities.
Prior art methods and apparatus for measuring coil sheet length using a measuring roll require that a non-slip condition be constantly maintained between the moving sheet and the circumferential surface of the measuring roll. Various arrangements of tachometers or rotary pulsers driven by the measuring roll and/or a coiler mandrel, and connected to either an analog or a digital electronics package, provide a pulse signal proportional to measuring roll circumference and/or number of coil wraps, and therefore proportional to coil sheet length.
As is often the case in sheet steel mill rolling practices, particularly in high speed mills, the non-slip condition between moving sheet and measuring roll circumference cannot always be maintained. There are a number of operator preferences and structural prohibitions that permit only a single billy roll installation as a universal solution to coil sheet length measuring problems in both new and old modified sheet rolling mills. Some of the prior art methods and apparatus tested under the slip conditions in mills produced coil sheet length errors as great as .+-. 2%. The resulting shortage alone amounted to considerable customer debits for a single year. In addition, none of the prior art arrangements provided slip compensation, nor correction for errors due to partial last wrap on the coiler, which on large diameter coils of about 5 feet or more may produce substantial errors in itself. Nor was correction made for still further errors produced by sheet thickness varying from a nominal value, except by actually gaging the moving sheet and feeding a corresponding signal into the electronics package for compensation purposes. Moreover, none of the prior art arrangements having a measuring roll and experiencing slippage permit reversal of the rolling mill sheet movement without producing substantially erroneous length data.